Hydrogen molecules and the radiative cooling of pregalactic shocks

The nonequilibrium radiative cooling, recombination, and molecule formation behind steady state shock waves in a gas of primordial composition have been calculated in detail for a number of cases. The authors have solved the rate equations for these processes, together with the hydrodynamical conservation equations. Such shock waves are relevant to a wide range of theories of galaxy and pregalactic star formation. A purely atomic gas of H and He which is shock-heated to temperatures above 104K is assumed. The results indicate that formation of H2 molecules in the post-shock gas may be quite common for a significant range of shock velocities. The extra cooling resulting from H2 formation greatly reduces previous estimates of the characteristic gravitational scale length and the characteristic mass subject to gravitational instability in these postshock regions.